Method and means for reducing cross talk in carrier-current signaling systems



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'Patented July 10, 1928,

1,676,627 PATENT oFFlcE.

HAROLD IBL-ACK, OF EAST ORANGE, NEW JERSEY, .AS-SIGNOR TO BELL TELEIHONELABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEWYORK.

MnTHonAND MEANS 'Fon REDUCING cnoss TALK 1N CARRIER-CURRENT sIGNALINaSYSTEMS.

.Application led August 30, i926. Serial No. 132,452.

This invention relates to methods and means for reducing crosstalkincarrier culrent signaling systems.

An object of the invention is to reduce 5 crosstall between adjacentsignaling systems.

A related object of the invention is to reduce reflection effects suchas are commonly caused by impedance irregularities in electric signalingsystems. 1

The invention provides a simple and efficient arrangement fior reducingcrosstalk between. adjacent carrier current signaling systems whichconsists in attenuating reiected waves at one terminal of each system tosuch an extent that their effect at a distant terminal is negligible,and also in maintaining the currents transmitted in the same directionin each system at substantially the same energy level at all pointswhere the transmission lines rup adjacent each other.

When two or more signaling systems are operated on the same pole line orin the same cable, one of the principal problems encountered is toreduce crosstalk or mutual interaction due to inductive or capacityeffects between adjacent systems.

In order to prevent crosstalk in voice frequency signaling syst-ems itis customary to transpose the pairs of wires at intervals which areshort relative to the wave lengths of the currents employed. This canusually be done without hardship in such systems since the frequenciesare relatively low and the wave lengths correspondingly long. In carriercurrent signaling systems, however, where the wave lengths involved arerelatively short, transpositions are required at much more frequentintervals and the problem of reducing crosstalk becomesA increasinglydifficult. y

It has been found that crosstalk can be materially reduced by reducingreflection effects which appear in the form of waves which are refiectedfrom terminal or re-l peater devices which anc connected toatransmission line but do not accurately match the impedance of the line.A method of reducing such reflection effects is dis closed, for example,in a copending application of C. W. Green, Serial No. 132,670, filedAug. 31, 1926, in which a loss element is-arranged to be traversed bythe refiected Wavesagreater number of times than by the signal, so thata discrimination is made in favor ot' the signal. One advantage of thismethod is that waves which are reflected back over one transmission linedue to impedance mismatching are attenuated to such an extent that -theycan produce only negligible crosstalk in an adjacent line.

According to a feature of the present invention, a loss element isinserted between atransmission line and a terminal grouping filter' orother device from which the crossta-lk components are reflected so thatthe crosstalk components will be greatly attenuated in traversing theloss element twice, once before they are reflected and again after theyare refiected. By thus reducing crosstalk refiection effects at oneterminal of a transmission line their effect at a distant terminal ismaterially reduced and the problem of transposing the line circuits isgreatly simplified. I

According to another feature of the invention, crosstalk is reduced bymaintaining similarly directed signaling'currents on adjacent lines atsubstantially the saine energy level. A special case of this to whichthe invention applies is where a number of carrier lines are carried ont-he same pole line and terminate at different geographical points. Forexample, in a single channel carrier currenttelephone system, such asthat hereinafter described, one such carrier line may extend from A toB, while another carrier line may come in on the same pole line at someintermediate point C and extend beyond the point B. According to thisinvention, crosstalk between such systems is reduced by employing thesame energy levels on the lines where they run adjacent each other. Thisis preferably accomplished by raising the energy level of one line byamplification at the transmitter and by lowering the energy level of theother line by a resistance pad at the transmitter, compensation beingeffected at the receiversby a pad and amplifier, respectively.

The invention will be described in connection with its application tosingle channel carrier current telephone systems, but it will be evidentthat it is also applicable to other types of signaling systems.

In the drawing:

Fig. l is a diagranmiatic illustration showing two single channelcarrier current telephone sytems having adjacent transmission lines,land provided with means for re- -ducing reflection from the terminalgrouping filters; and

Fig. 2 is a diagrammatic illustration showing two similar systems, inwhich inutual interaction is reduced by employing the same energy levelson both transmission lines where they rnn adjacent each other.

In Fig. 1,` the terminal circuits of two single channel carrier currenttelephone systems, located at stations 1V and E, are interconnected bythe main transmission lilies ML1 and ML2, respectively, which arelocated on the same pole line or in the saine cable. The single channelsystems per se form no part of the present invention, but may be of thetypes disclosed in U. S Patent No. 1,602,019, issued Oct. 5, 1926 to C.L. lVeis, Jr., or in my U. S. Patent No. 1,653,837, issued Dec. 27,1927.

The two systems shown in Fig. 1 may be similar in all respects and henceonly the circuits associated with the line MLl will be described in.detail. This system isv composited for simultaneous low frequency andcarrier current communication. This is accomplished by carrier compositesets, the set at station 1V comprising the voice frequency filter VF andthe carrier frequency filter OF, and the set at station E comprisingsimilar filters VF and OF. VThe filters VF and VF are positioned in thevoice frequency telephone branch, and the filters OF and CF in thecarrier current branch, of the transmission line. These filters serve toseparate the currents corresponding to the frequencies used in theirrespective types of transmission. Obviously, the system could be usedfor carrier transmission only, in which case these filters could beeliminated.

The filters of the carrier composite sets, described above, and each ofthe other filters shown throughouttlie systems may be dcsigned inaccordance with the principles set forth in U. S. Patent No. 1,227,113to A. Campbell, issued May 22, 1917.

The terminal apparatus at station 1V includes a transmitting channel TCand a receiving channel RO. and the terminal apparatus at station Eincludes a transmitting channel TC and a receiving channel RC. Carriercurrents of different frequencies are utilized for transmission inopposite directions over the line MLl, the currents transmitted', fromstation E to station 1V being chosen of higher frequency than thecurrents transmitted `from station 1V to station E.

The transmitting channel TC includes an oscillator-modulator OM and a10W frequency band filter BF while the receiving channel RC includes a10W pass filter LP, an

oscillator-detector OD and a high frequency band filter BF1. The filtersBF and BF1 serve to separate the directional bands of frequencies to therespective terminal transmitting and receiving channels, the'lower bandof carrier frequencies being assigned to channel TC and the upper bandof frequencies being assigned to channelRC. The corresponding elements.at station E are designed by similar reference characters with primesalliXe-d thereto.

A lowv frequency line L, which may be an ordinary subscriber-s telephoneline is associated with channel TC-RC at station "W for communicationover the line MLl With a similar low frequency line L associated withchannel 'PU-RC at station E. The low frequency lines L and L areprovided with balancing artificial lines or networks N and Nrespectively, and with differential repeating coils H and H, commonlyknownl as hybrid coils, for enabling independent transmission in tivodirections between the lines and theterminal circuits.

Voice frequency currents originating in thc low frequency line L atstation WV pass through the associated hybrid coil H into theoscillator-modulator OM where they are combined with carrier currents ofthe frequency assigned to channel TC. Of the components of modulationappearing in the output circuit of the oscillator-modulator OM, the lowfrequency band filter BF suppresses all but one side band, for example,the lower side band, which it'transmits or passes to the transmissionline MLl.

rl`he side band currents incoming at station E pass through the lowfrequency band filter B191 in the receiving channel RO', and aredetected in the oscillator-detector OD. Voice frequency components ofdemodulation appearing in the output circuit of oscillator-detector ODare selectively transmitted by the low pass filter LP and pass throughhybrid coil H to the low frequency line L. Transmission from station Eto station W is accomplished in the same manner, except that higherfrequency carrier currents are employed.

1n order to prevent near end crosstalk between ,transmission lines ML,and ML2, the Iarrier currents utilized for transmission in eachdirection are of the same frequency in both systems. That is, thecurrents transmitted from station 1V to station E are of the samefrequency in both systems, and the currents transmitted from station Eto station 1V are likewise of the saine frequency in both systems. Itwill thus be seen that even though a signal modulated carrier Wave ofthe lower frequency band transmitted over line MLg from station Wappears in line ML,L as crosstalk, currents of this frequency are notpassed by the band filter BFI, in the receiver channel RO and', ofcourse, cannot pass backwards through the transmitting channel TC. Thisfrequency allocation likewise prevents objectionable'near end cross talkfrom line- ML, into line ML2. Y

If, however, impedance irregularities exist in the terminal apparatus atstation W', for

example, if the band filter BF is improperly termina-ted, signalmodulated waves following the path from line ML2 to line ML, and meetinga change in impedance in the filter BF will be refiected over the lineML, and, beine' readily transmitted by the band filter BF, will appearin the receiving channel RC as far end Crosstalk. The same effect isproduced when a portion of the lower frequency signalmodulated carriercurrent transmitted over line ML2 from station W to station E is reectedfrom the low frequency band filter associated with the line ML2 atstation E, taking the path 6 directly into the band filter EF1. SimilarCrosstalk may occur in' the upper frequency band transi mitted in theopposite direction.

It is very difficult to prevent reflection in the terminal groupingfilters for the reason that only a slight error in the impedance of theelements of a filter is sufficient to prO- duce considerable refiection.It is practically impossible with present manufacturing methods to buildcommercial condenser elements to closer than 0.8% accuracy or to windcoils to closer than 1.2% accuracy, making a total of 2% gross error inthe filter elements. This is sufficient to produce an impedanceirregularity of as much as 60% near the edge of the frequency scale ofthe filter.

In order to compensate for the effect of improper filter termination,acording to this invention, a resistance pad K is inserted bctween theterminal grouping filters and the carrier composite set at station W anda similar resistance pad K is inserted between the terminal groupingfilters and carrier composite set at station E. Similar resistance padsK2 and K2 are provided -in the line ML2. With the pad Kin the positionshown, the energy which is reflected passes through the pad twice, oncein each direction, whereas the outgoing and incoming signaling currentspass through the pad but once. This produces a definite discriminationbetween Crosstalk components and the signaling components. Theresistance pads may be of the type disclosed in U. S. Patent Np.1,591,073 issued July 6, 1926 to O. J. Zobel, having a constantresistance characteristic impedance suitable for terminating the carriercircuits.

Where a large number of single channel systems are operated on the samepole line it is y'often desirable to terminate the systems at differentgeographical oints, as illustrated in Fig. 2. The line M in Fig. 2, forexample, may extend from A. to B, a distance of' say 200 miles, whilethe line ML2 may come in at some intermediate point C and extend to thepoint D, some dlstance beyond the point B. Crosstalk between thesesystems is materially reduced by employing the same energy level forsimilarly directed currents on the lines ML, and ML2 where they runadjacent each other. This is accomplished by inserting level adjustingpads in certain instances on the drop sides of the carrie-r terminalapparatas, and by the use of terminal amplifiers to control the gainlevels. At the terminal A, an amplifier G is inserted in thetransmitting -channel TC bet-Ween the two sections of a split bandfilter BF to raise the outgoing currents to the desired level, and att-he terminal B a resistance pad P is inserted between the two sectionsof a similar band filter BF, to reduce the level of the incomingcurrents to the desired value for reception. At the terminal C aresistance pad P2 is inserted between the sections of the band filterBF2 and serves to bring the energy level of the currents transmitted tothe line ML2 down to the point whe-re it is equal to the energy level ofthe currents transmitted in the same direction over the line ML, at thepoint X where the line ML2 comes in on the same pole line with the lineML,. At the terminal D an amplifier G2 is inserted between the sectionsof filter BF3 in the receiving channel RC2 to raise the level of theincoming currents which were reduced at station C in the manner de-4scribed. Since this. terminal 4is at the far end of the pole line, anlamplifier G2 is included in the transmitting channel TG2 to raise theoutgoing currents to the desired level, and a resistance pad P2 similarto the pad P at terminal B is inserted in the receiving channel RC2 atterminal C. A resistance pad P is also provided in the channel TC atterminal B to bring t-he energy level of the currents transmitted to theline ML, down to the same level as similarly directed currents at thepoint Y on the line ML It2 will be noted that in carrying out theinvention described inconnection with Fig. 2, the upper bands of carriercurrents are transmitted in the same direction over both transmissionlines, while the .lower bands' of carrier currents are transmitted 4inthe opposite direction. It is therefore part in the termination of themain transmission lines. These pads and the gain adjusting amplifiersare preferably variable in order to renderthcm suitable for universalapplication. These elements may therefore be appliedto existing systemsto control the energy levels without otherwise changing the circuits. Inaddition to these pads and amplifiers, the resistance pads K, K', etc.,described in connection with Fig. l, are also used to reduce reflectiondue to mis-matching of terminal impcdances.

The invention is also capable of other modifications and adaptations notspecifically referred to but included Within the scope of the appendedclaims.

What is claimed is:

l. The method of reducing crosstalk between adjacent signaling circuitswhich comprises attenuating the crosstalk components at the terminals ofsaid circuits a. greater number of times and to a greater extent thanthe effective signaling currents therein.

2. The method of reducing crosstalk be- Ytween adjacent signalingcircuits in which crosst-alk currents are reflected at a terminal, whichcomprises attenuating the reflected currents at said terminal to agreater extent than the signaling currentstraversing the circuit` toreduce the effect of the reflected currents at a distant terminal.

3. The method of reducing crosstalk between adjacent carrier currentsignaling circuits in which crosstalk currents are rel flected at oneterminal, which comprises attenuating the reflected currents a greaternumber of times and to a greater extent than the effective signalingcurrents transmitted from said terminal, to reduce the effect of saidreflected currents at a distant terminal. l

- 4. The method of reducing crosstalk between adjacent carrier currentsignaling systems which comprises maintaining the currents transmittedin the same direction in both systems at substantially the same energylevel.

5. The method of reducing crosstalk between carrier current signalingsystems having transmission lines which are adjacent each other for onlya limited portion of their length, which comprises maintainingsubstantially the same energy level of tr'ansmission on adjacentportions of said transmission liues.

6. The method of reducing crosstalk between carrier current signalingsystems employing substantially the same frequencies and energy levelsof transmission in the respective transmitting circuits and havingtransmission lines which are adjacent each other for only a limitedportion of their length,-

which comprises reducing the energy level of the currents at onetransmitting terminal to substantially the same value as the current ata point intermediate the terminals of an adjacent transmission line.

7. The method Vof reducing crosstalk between carrier current signalingsystems emplo ing substantially` the same frequencies an energy levelsof transmission in the respective transmitting circuits and havingtransmission lines which are adjacent each other for only a limitedportion of their length, which comprises reducing the energy level ofthe currents at one transmitting terminal to substantially the samevalue as the currents at a point intermediate the terminals' of anadjacent transmission line, and amplifying the currents received at thedistant terminal of said firstf mentioned transmission lineto compensatefor the reduction in energy level at the transmitting terminal thereof.

8. In a system fortransmitting Waves of different frequencies, atransmission line, a terminal circuit including a filter connected tosaid line causing certain waves to be reflected, and a loss elementbetween said'circuit and said line to attenuate the reflected waves.

9. In a system for transmitting Waves of different frequencies, atransmission line, a terminal circuit containing an impedanceirregularity connected to said line, and a loss element between saidcircuit and said line to reduce reflection caused by the misn'iatc-hingof terminal impedances.

10. In a system for transmitting Waves of different frequencies, atransmission line, a terminal circuit containing a frequencydiscriminating device connected to said line and having an impedancewhich differs from. that .of the line at certain frequencies, and a losselement between said circuit and said line to duce reflection from saiddevice.

11. In a system for transmitting waves of different frequencies, atransmission lino, terminal transmitting and receiving circuitsconnected to sail line, grouping filters in said circuits forselectively transmitting directional bands of frequencies, said filterscon` taining impedance irregularities causing waves in a portion of thefrequency band to be reflected, and a resistance pad between saidcircuits and said line to reduce the rcflected waves..

lf2. The combination with a pair of carrier-current signaling systemshaving adjacent transmission lines employing substantially the samedirectional bands of carrier frequencies of means associated with eachline to maintain the currents transmitted in one direction thereover atsubstantially the same energy level as the currents transmitted in thesame direction on the other line.

13. The combination with a pair of carrier current signaling systemsemploying substantially the same frequencies and energy levels oftransmission in the respective transmitting circuits and havingtransmission lines which are adjacent each other for only a limitedportion of their length, of a resistance element in the transmittingcircuit of one system to reduce the energy level of the currentsimpressed upon the associated transmission line to substantially thesame value as the currents at the same geographical point on the othertransmission line.

14. The combination with a pair of carrier current signaling systemscomprising terminal transmitting and receiving circuits and employingsubstantally the same frequencies and energy levels of transmission in fthe respective transmitting circuits, of transmission lines which areadjacent each other for only a limited portion of their length, aresistance pad in the transmitting circuit of each system which isadjacent the transmission line of the other system to reduce the energylevel of the currents impressed upon each transmission line tosubstantially the same value as the currents at the same geographicalpoint on the adjacent transmission line, and amplifiers in theassociated receiving circuits of said system to compensate for thereduction in energy in the transmitting circuits thereof.

In Witness whereof` I hereunto subscribe my name this 27th day of AugustA. D.,

HAROLD s. BLACK.

